Pluto
Yes, Pluto is really far out, from the Sun that is. Pluto is smaller than seven of the solar system's moons: Luna, Io, Europa, Ganymede, Callisto, Titan and Triton). In Roman mythology, Pluto (Greek: Hades) is the god of the underworld. After many suggestions maybe the planet received this name because it's so far from the Sun that it is in perpetual darkness or maybe because "PL" are the initials of Percival Lowell. Here are some of it's statistics:
| Distance from Sun: | |
|---|---|
| Mean: | 39.53 AU |
| Perihelion: | 4,434.99 x 106 km |
| Ahelion: | 7,304.33 x 106 km |
| Size: | |
| Volume: | 0.715 x 1010 km3 |
| Diameter: | 2390 km |
| Mass: | 1.25 x 1022 kg |
| Motion: | |
| Year: | 248.53 E Years |
| Day: | 6.4 E Days |
| Other: | |
| Density: | 1750 kg per m3 |
| Surface Temperature: | -220°C or 53°K |
Pluto was discovered in 1930 by a fortunate accident. Calculations which later turned out to be in error had predicted a planet beyond Neptune, based on the motions of Uranus and Neptune. Not knowing of the error, Clyde W. Tombaugh at Lowell Observatory in Arizona did a very careful sky survey which turned up Pluto anyway.
After the discovery of Pluto, it was quickly determined that Pluto was too small to account for the discrepancies in the orbits of the other planets. The search for Planet X continued but nothing was found. Only in 2003 were new Planetary bodies discovered beyond Neptune.
From it's discovery in 1930 till 2006, Pluto was considered to be a Planet. The International Astronomical Union, in order to better classify bodies in the solar system has reclassified Pluto as a Dwarf Planet. This decision, though controversial, prevented many more recent discoveries to be classified as planets. This is especially important when it is estimated that as many as 100 Pluto like bodies exist beyond Neptune's orbit.
Fortunately, Pluto has a satellite, Charon. By good fortune, Charon was discovered (in 1978) just before its orbital plane moved edge-on toward the inner solar system. It was therefore possible to observe many transits of Pluto over Charon and vice versa. By carefully calculating which portions of which body would be covered at what times, and watching brightness curves, astronomers were able to construct a rough map of light and dark areas on both bodies. See Pluto rotate (left).
One of the main reasons Pluto is no longer considered a planet is because of Charon. You see, Charon is not a satellite of Pluto. The point that they both orbit around is in between the two making them orbit around each other. Since a planet does not share it's orbit this disqualifies Pluto as a planet. Bummer.
Pluto's radius has not been accurately measured yet. JPL's value of 1137 is given with an error of +/-8, almost one percent.
Little is known about Pluto's atmosphere, but it probably consists primarily of nitrogen with some carbon monoxide and methane. It is extremely tenuous, the surface pressure being only a few microbars. Pluto's atmosphere may exist as a gas only when Pluto is near its perihelion; for the majority of Pluto's long year, the atmospheric gases are frozen into ice. Near perihelion, it is likely that some of the atmosphere escapes to space perhaps even interacting with Charon.
There is only one mission to Pluto and it hasn't even gotten there yet. The New Horizons probe is screaming toward Pluto at 10 miles per second and is scheduled to arrive July 14, 2015.
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